Viruses cause many different diseases in man and animals. For viruses to infect cells in culture (and presumably to infect target cells within the host) they must first adsorb to and penetrate the target cells. Studies of virus adsorption in vitro have led to the finding that viruses adsorb to specific receptors on the surface of cells in culture. The presence of specific receptors is also a key determinant of tisse tropism in vivo. The biochemical nature of these receptors is poorly understood. This proposal is directed toward the elucidation of the structure of reovirus receptors as one model of animal virus receptors. Reoviruses have been shown to bind to specific receptors on several types of mammalian cells in culture. To gain insight into how reoviruses penetrate host cells we are studying the interaction between virions or purified viral cell attachment polypeptide (Sigma 1) and specific receptors on mouse L cells. Our experimental approach involves: (a) purification of the reovirus Sigma 1 polypeptide by affinity chromatography on anti-type 3 Sigma 1 lgG-sepharose columns. (b) Characterization of the binding reaction between Sigma 1 polypeptide and mouse L cells. (c) Characterization of the biochemical groups necessary for binding to membrane receptors. (d) Isolation of receptor negative reovirus resistant L cells using a reovirus top component-diphtheria toxin fragment A hybrid toxin. (e) Isolation of reovirus sensitive revertants (receptor positive cells) from the reovirus resistant L cell mutants. (f) Biochemical identification and systematic characterization or receptors on sensitive, resistant strains of L cells. Two novel biochemical cross-linging procedures will be developed which may allow identification of Sigma 1-receptor complexes and biochemical characterization of receptors on sensitive and resistant L cell strains. (g) Development of methods for the solubilization and partial purification of biologically functional receptors using reovirus top component as an affinity absorbent.